This invention relates to a filter bed for use in conjunction with a granulation facility for blast furnace slag. More particularly, this invention relates to a new and improved filter bed wherein the layers of filtering material (gravel) are provided in individual containers preferably comprised of sheet metal (steel) which are closed on all sides and placed in abutting or adjacent relationship in the filter bed. These containers all include removable covers with the covers, bottoms of the containers and sides of the containers all being perforated.
It is well known that during the processing (granulation) of blast furnace slag, the molten slag material is passed through a filter bed for removal of impurities from the slag. The core section of such a filter bed is comprised of a plurality of filter layers which are formed by pouring several layers of solid matter, preferably sand, on top of one another. Typically, three or more grain sizes of sand are used in the filter bed with the lower most layer consisting of the group of largest grain size and the upper most layer consisting of the group of smallest grain size. Thus, for example, an upper sand layer may have a grain size of between about 4-8 mm, with middle layers having sand with grain sizes of between about 8-16 mm and the lowest layer of sand having a grain size of between about 16-25 mm.
Significantly, this layered arrangement of varyingly sized grains of sand is used in the actual operation of a filter bed as much as possible. This is because the lower layer of sand contains finely perforated pipes embedded therein which serve to carry off the filtrate (liquid). These finely perforated pipes have a tendency to clog or stop up due to insufficient filtering whereby fine grain sand and other particles travel to the lower layers from the upper layers and clog up the perforated pipe. In order to maintain the effectiveness of the filtering layer on a continuous basis, and also in order not to obstruct the perforated drain pipes, the impurities (that is, the fine slag and sand particles) are removed from the layers of sand by the use of compressed air being blown therethrough. This compressed air is blown through the built up filter layers in a periodically repeating cycle using a reverse current. Unfortunately, this compressed air current can disturb the desired multilayer arrangement; and can also be unsuccessful in removing the fine impurities from the layers of sand. If this purification measure i.e., the use of compressed air, fails the only alternative is to remove the entire sand mass from the filter bed and lay down new layers of sand with new drain pipes.
The laying down of a new filter bed as discussed above is a protracted process and requires care and skill, particularly in the pouring of the sand layers of different grain sizes on top of each other. It will be appreciated that formation of a new filter bed is especially undesirable if the new filter bed has to be prepared during the filtration and granulation operation itself. Thus, if one has to interrupt the entire operation and prepare a new filter bed due to problems such as pipe blockages or erosions which appear in certain filter bed zones, the result is both expensive and labor intensive. Moreover, during peak periods, operators are often forced to repair the filter bed in a prophylactic manner, since the above described problems with the filter beds typically arise suddenly during use; while the repair thereof often takes weeks.